1. Field of the Invention
This invention relates to time-indicating labels. In one aspect the invention relates to multilayer, dye-based labels while in another aspect, the invention relates to labels comprising a top sheet, activating layer, timing layer, dye and a substrate.
2. Description of the Related Art
Numerous devices are known which provide, after activation, a visual indication of the passage of a predetermined period of time. Such time indicators are useful, for example, as a security badge, as an indicator of the length of time a perishable item has been on the wholesaler's or retailer's shelf, and for numerous other uses.
One problem that generally exists with such a time indicators is that they gradually change color over a period of time and it is difficult to ascertain the actual completion of the time interval, i.e., it is a “grey” time interval. For a time indicator to be useful, its appearance should not change for at least 70% of its intended life. Such a time indicator would remain until near the end of the time interval for which it was created, and then the color (or image) rapidly would appear. In essence, what is desirable is a time switch meaning a rapid color change to clearly show that the time interval has ended.
U.S. Pat. No. 5,822,280 and the references cited within it describe various attempts at devising time indicators that provide this step function. While most perform in an acceptable manner, all are subject to improvement.